1. Technical Field
The present invention relates to an electro-optic element and a scanning optical device.
2. Related Art
In recent years, a scanning image display device for displaying an image by raster-scanning a light beam such as a laser beam on an irradiated surface has been proposed. In this device, high contrast display is possible in comparison with a projector or the like using, for example, a liquid crystal light valve because complete black color can be displayed by stopping supply of the laser beam. Further, since an image display device using a laser beam has characteristics such that the single-wavelength laser beam causes high color purity, and that the high coherence thereof makes the shaping (aperturing) of the beam easy, the image display device using the laser beam is expected to be a high-quality display achieving a high resolution and high color reproducibility. Further, since the scanning image display device does not have any fixed pixels unlike with a liquid crystal display, a plasma display, and so on, the scanning image display device does not have a concept of the number of pixels, and consequently, has an advantage that the resolution can easily be converted.
In order for generating an image by the scanning image display device, it is necessary to scan a light beam two-dimensionally using a scanner such as a polygon mirror or a galvanometer mirror. Although a method of scanning the light beam two-dimensionally by swinging a single scanner in two directions, namely a horizontal direction and a vertical direction can be considered, in such a case, there arises a problem that the structure and control of the scanning system become complicated. Therefore, there has been proposed a scanning image display device provided with two sets of scanners each for scanning a light beam one-dimensionally and arranged to have charges of horizontal scanning and vertical scanning, respectively. In the past, it has been common to use polygon mirrors or galvanometer mirrors for both of the scanners, and a projection device using rotating polygon mirrors for both of the scanners is disclosed in JP-A-1-245780.
However, although in the above document the device using the polygon mirrors is introduced, the scanning frequency increases with increase in resolution of the image format, and approaches the limit with polygon mirrors or galvanometer mirrors. Therefore, in recent years, a system applying the microelectromechanical systems (MEMS) technology to the higher-speed scanner has been introduced. A scanner (hereinafter simply referred to as a MEMS scanner) applying the MEMS technology denotes a scanner manufactured using a fine processing technology of a semiconductor material such as silicon, in which a mirror supported by a torsion spring and so on is driven with electrostatic force. This scanner can scan a light beam by reciprocating the mirror with the interaction of the electrostatic force and the restoring force of the spring. By using the MEMS scanner, a high-frequency scanner with a large deflection angle in comparison with past scanners can be realized. Thus, it becomes possible to display a high-resolution image.
Incidentally, since the reciprocating motion of the mirror at the resonance frequency is required for realizing a high-speed MEMS scanner, in consideration of efficiency of a light beam, there is adopted a system in which a scanning line is formed from left to right viewed from the viewer, and then the next scanning line is formed from right to left (bi-directional scanning).
On the other hand, since the image signal is standardized based on cathode ray tubes (CRT), the format thereof corresponds to scanning (unidirectional scanning) in which the scanning point moves from left to right, then returns to left in a short period of time, and moves to right again. Therefore, in the case with the MEMS scanner, regarding a part of the data, the order of the signals needs to be reversed when displaying the data, and consequently, the control of the signals becomes complicated.
Consequently, as a scanning method other than the MEMS scanner, there can be cited an electro-optic (EO) scanner. The EO scanner is an element in which the proceeding direction of a light beam transmitted through an EO crystal is changed by applying a voltage to the EO crystal. As described above, since in the EO scanner the scanning angle can be controlled by the voltage, drawing with the unidirectional scanning becomes possible similarly to the CRT.
Further, in the EO scanner, electrons are injected in the EO crystal by applying the voltage thereto, thus unevenness is generated in the electron distribution. Therefore, the distribution is generated in the refractive index alteration caused by a Kerr effect to deflect the incident light beam towards the side with a higher refractive index, thus making the scanning of the light beam possible. Further, since the gradient of the refractive index distribution inside the EO crystal depends on the amount of the injected electrons, namely the applied voltage, by varying the applied voltage, the scanning angle of the light beam emitted from the EO crystal can be controlled.
Incidentally, in a display device using the EO scanner, it is necessary to modulate the intensity at positions corresponding to respective pixels in order for displaying the image. In the case in which Super Video Graphics Array (SVGA) class display is performed, the modulation rate higher than about 30 MHz is necessary. However, depending on the type of the laser source, there arises a problem that the sufficient modulation rate can hardly be obtained. Further, although it is also possible to provide an external modulator using an electro-optic element or an acousto-optic element separately from the EO scanner, there arises a problem that increase in the number of elements incurs rise in cost, and further, requires an additional volume resulting in growth in size.